The Unselective Net: How Fishing Gear Is Shaping the Fate of Bigeye Tuna
Exploring the relationship between purse-seine fishing gear and the unintended capture of vulnerable bigeye tuna in the Eastern Pacific Ocean
The Invisible Problem Beneath the Waves
Imagine a wall of netting, deep enough to engulf the Empire State Building and long enough to span dozens of city blocks, being deployed around entire schools of fish in the open ocean. This is the reality of modern purse seine fishing in the Eastern Pacific Ocean, an industrial process that supplies the world with canned tuna but poses a complex challenge for marine conservation 6 . While efficiently catching target species like skipjack and yellowfin tuna, these enormous nets inadvertently capture vulnerable species, including the deep-dwelling bigeye tuna (Thunnus obesus) 2 .
The survival of this ecologically and commercially valuable species depends on a surprising factor: the specific characteristics of the fishing gear itself. Understanding this relationship is not just academic—it is crucial for designing smarter fishing practices that can feed humanity without emptying the oceans.
What Is a Purse Seine and Why Does It Catch More Than It Should?
The Anatomy of a Modern Fishing Method
A purse seine is essentially a gigantic curtain of netting used to encircle entire schools of fish. The process gets its name from the "purseline" – a cable that runs through rings along the bottom of the net. Once a school is surrounded, the purseline is hauled in, drawing the bottom of the net closed like an old-fashioned purse, trapping everything inside 6 .
The method's efficiency depends heavily on how fishers locate their catch. There are three primary set types, each with dramatically different environmental impacts:
Historically, fishermen located tuna by chasing dolphin pods, with which tuna often associate. This method led to significant dolphin mortality and is now heavily regulated.
This modern approach targets tuna congregating around Fish Aggregation Devices (FADs) – both natural (like logs) and human-made. This method is particularly problematic for bigeye tuna.
This involves targeting free-swimming tuna schools not associated with any object or marine life, resulting in relatively pure catches of surface-schooling species like skipjack.
The Bigeye Tuna's Dilemma
Bigeye tuna face a unique vulnerability due to their biology. Unlike their skipjack and yellowfin cousins that prefer warmer surface waters, adult bigeye tuna are deep-water specialists. They spend much of their time in cooler, deeper layers, only coming to the surface at specific times, often around floating objects like FADs 7 .
This is where the gear characteristic becomes a critical life-or-death factor. When a purse seine net is set around a FAD, it captures not just the surface-schooling skipjack but also the deep-dwelling bigeye that have been attracted to the same structure. Because bigeye tuna are less abundant and mature more slowly than other tropical tunas, this unintended catch can have severe consequences for their population.
The Scientific Toolkit: Tracking a Fishery's Footprint
How do we know all this? The story of bigeye tuna and purse seines is pieced together through rigorous, long-term scientific monitoring. The primary data comes from a comprehensive system established in the Eastern Pacific.
Key Research Methods and Data Sources
| Data Source | What It Tracks | Collected By | Since |
|---|---|---|---|
| Vessel Logbooks | Catch amounts by species, fishing location, set type | IATTC from fishers | 1952 1 |
| Onboard Observers | Independent verification of bycatch (dolphins, turtles, sharks) and fishing practices | IATTC & NMFS observers | Ongoing program 1 |
| Landings Data | Weights and species composition of final catch at port | IATTC | 1952 1 |
Stock Assessment Models
Scientists feed the collected data into complex computer models to estimate population trends for bigeye tuna and determine whether current fishing levels are sustainable.
Electronic Monitoring
Increasingly, cameras and sensors on vessels provide additional data on fishing operations and bycatch, complementing human observer programs.
A Tale of Two Catches: Data Reveals the FAD Problem
The core of the issue is revealed when we compare what is caught in different types of sets. Data aggregated by bodies like the IATTC show a clear and consistent pattern: sets made on floating objects (FADs) catch a much higher proportion of non-target species, including bigeye tuna and vulnerable sharks and rays, compared to sets on free-swimming schools.
The following chart illustrates a simplified, hypothetical catch composition based on typical fishery data, showing why FAD sets are the primary concern for bigeye tuna bycatch.
| Set Type | Skipjack Tuna (metric tonnes) | Yellowfin Tuna (metric tonnes) | Bigeye Tuna (metric tonnes) | Sharks & Rays (metric tonnes) |
|---|---|---|---|---|
| Floating Object (FAD) Set | 180,000 | 31,000 | 15,000 | 1,500 |
| Free-Swimming School Set | 170,000 | 40,000 | 500 | 100 |
Engineering Solutions: The Toolkit for a Smarter Fishery
The scientific diagnosis of the problem has spurred a wave of innovation aimed at making purse seining more selective. Researchers, conservation groups, and forward-thinking fishers are collaborating on a suite of technological fixes. These solutions focus primarily on modifying the two key elements: the FADs themselves and the fishing procedures.
| Solution | How It Works | Benefit for Bigeye Tuna |
|---|---|---|
| Non-Entangling, Biodegradable FADs | Replaces netting with non-entangling cloth and uses biodegradable materials like bamboo. | Eliminates entanglement risk for turtles and sharks. Biodegradable structure reduces marine debris and "ghost fishing" if lost 6 . |
| Backdown Method | After setting the net, the vessel tows it to create an opening at the surface. | Allows trapped dolphins, sea turtles, and other non-target species to escape 6 . |
| Non-Dolphin/Whale Shark Sets | A procedural rule avoiding setting nets on marine mammals or whale sharks. | Directly prevents bycatch of these charismatic and vulnerable species 6 . |
| Modified Purse Seines (for seabirds) | Uses larger mesh sizes and removes excess netting material at the surface. | While designed to save seabirds from entanglement, it simplifies the gear and can reduce bycatch overall 6 . |
Biodegradable FADs
Traditional FADs made with synthetic materials can persist in the ocean for years, continuing to attract and trap marine life. Biodegradable alternatives break down naturally if lost or abandoned.
Escape Mechanisms
Innovative net designs with escape panels or modified mesh sizes allow smaller, non-target species to exit the net before it's fully closed and hauled aboard.
A Net Gain for the Future
The story of bigeye tuna in the Eastern Pacific is a powerful reminder that the solution to environmental challenges often lies not in stopping an activity altogether, but in refining it. The relationship between gear characteristics and the presence of bigeye tuna in purse-seine catches is no longer a hidden mystery but a well-documented scientific fact. This knowledge has empowered the fishery to begin a technological evolution—from crude, non-selective nets to smarter, more accountable systems.
The path forward is clear. It involves supporting the continued adoption of non-entangling, biodegradable FADs, perfecting escape mechanisms for bycatch, and maintaining rigorous scientific monitoring 6 . By continuing to innovate and refine the tools of the trade, we can steer toward a future where the tuna in our sandwiches comes from a fishery that actively protects the deep-dwelling bigeye and the rich ocean ecosystem it calls home.
The net that fishes smarter, not just harder, is the net of the future.